Domestically produced molten salt valves for photothermal power generation
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This post was last edited by Desert Fish on 2021-8-1 09:12. As a key component in concentrated solar power plants, molten salt valves must operate properly under complex conditions to ensure the safety and reliability of the entire system. It is necessary to avoid issues such as valve leakage, fluid retention, sticking during opening and closing, and corrosion; therefore, very high standards are required regarding the quality of high-temperature molten salt valves. Due to its technical complexity, coupled with the fact that the domestic market has never developed to date, the market for molten salt valves used in concentrated solar power generation has always been dominated by foreign companies. To further reduce costs and increase the level of localization of solar thermal equipment, melt salt valve manufacturers, like other suppliers in the industry chain, have invested substantial amounts in research and development over the years to significantly improve the quality and performance of their melt salt valve products. Residue-proof molten salt valve
Large-differential-pressure molten salt control valve for tower-type concentrated solar power plants
Both the binary molten salt (60% sodium nitrate + 40% potassium nitrate) and the ternary molten salt (53% potassium nitrate + 40% sodium nitrite + 7% sodium nitrate), which are widely used in concentrated solar power generation systems, contain certain amounts of highly corrosive chemical components. As working media, they feature high operating temperatures, high freezing points, and large variations in operating temperature differences. As a key component in energy storage systems, the molten salt valve plays a crucial role in ensuring the safety and reliability of the entire system’s operation. Which type of valve is more suitable for photothermal molten salt heat storage systems? How to deal with melt salt valve leakage and freezing blockages? What key factors need to be considered to produce qualified molten salt valve products under the harsh operating conditions of high-temperature molten salt heat storage systems? How can the cost of molten salt valves be reduced? What impact will the differences in technology and price between domestic and foreign molten salt valve products have on the procurement for China’s first batch of solar thermal demonstration projects?
As molten salt heat storage becomes increasingly widespread in the concentrated solar power industry, the importance of molten salt valves becomes evident. A quality issue with just one of these valves in a system can affect the proper operation of the entire power plant, resulting in incalculable losses. Some experts even suggest installing two molten salt valves, so that if one fails, the other can take over, which underscores the significance of molten salt valves in heat storage systems. Due to the characteristics of molten salt systems, the molten salt valves used in solar thermal power plants (including some products from foreign manufacturers) generally face technical problems such as freezing and blockage, corrosion, sticking at high temperatures, and cavitation. After the molten salt solidifies, it can cause valves to fail to open or close, and may even damage the packing and bellows; therefore, molten salt valve manufacturers around the world have conducted extensive research on the design and selection of packing. Wu Junjie, the marketing manager of Suzhou Antwei Valve Co., Ltd. (hereinafter referred to as Suzhou Antwei), told reporters that regarding the issue of damaged packing, the most critical techniques involve conducting accurate calculations of the heating heat field and the cooling heat field for the valve body, carrying out effective experimental verification, and selecting an appropriate heating control scheme. Nitrate molten salts react with graphite through oxidation, which limits their use in photothermal power plants.
To design and produce qualified molten salt valves, the following key points need to be considered: 1. The material selection must ensure high-temperature resistance as well as appropriate corrosion resistance. 2. Control of raw materials, with the quality of castings and forgings meeting the corresponding standard requirements. 3. The valve structure prevents the formation of dead zones, ensuring necessary fluidity. 4. The dynamic seal of the valve stem adopts a dual-seal design consisting of a bellows and a high-temperature resistant packing. 5. The hardness of the sealing material covering the valve seal pair meets the corresponding standard requirements. 6. The material chosen for the valve stem must be resistant to high temperatures and corrosion, as well as possess good chemical stability under high-temperature conditions. 7. Avoid the impact of bolt creep under high-temperature materials and alternating temperature conditions on the sealing performance. (For example, the flange fasteners in the products manufactured by Beijing Valve Factory are tightened using a special pre-tightening method to prevent the bolts from automatically loosening due to thermal expansion and contraction) ； For the dynamic seal of the valve stem, a combination of Inconel bellows and high-temperature resistant mica packing is used to create a dual seal, ensuring reliable sealing. ) 8. To prevent freezing and blockage in valves, an electric heat tracing system is required.
The molten salt valves currently used in this project generally have several issues, specifically including: 1. Residual molten salt remaining inside the valve body, causing internal leakage in the valve； 2. Molten salt crystallization at the upper part of the bellows ； 3. The filler box is corroded ； 4. Stem deformation and sticking at high temperatures. The stem of a control valve is usually much thinner than that of a shut-off valve. Based on experimental experience, it is believed that since the stem of a control valve is thin and subject to thermal cycling, careful consideration must be given when selecting the material for such stems ； 5. Molten salt cavitation phenomenon. Regarding cavitation, if the pressure difference across the valve exceeds the maximum saturated pressure difference of the medium (△Pmax), flashing occurs, which in turn leads to cavitation and causes damage to the interior of the valve as well as the adjacent pipe structures. A simple way to determine whether cavitation will occur is to check if the value of P1-P2 is less than the maximum saturated vapor pressure of the molten salt (△Pmax); if so, cavitation will not take place. So what is the saturated vapor pressure of molten salts? This requires valve manufacturers to collaborate with research institutions that have the necessary capabilities to conduct tests. Without this parameter, the only option is to choose based on the valve’s structure and improve the mechanical properties of the material to prevent cavitation.